vi verner liv, miljø og verdier... 1 gas detection systems sensor operating theory
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Gas Detection SystemsGas Detection Systems
Sensor Operating Theory
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Gas/Vapor Hazards-Gas/Vapor Hazards-
• Flammable/Combustible• Toxic• Oxygen-deficiency• Oxygen-enrichment
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Gas Detection Objectives Gas Detection Objectives
• Measure gas before hazardous concentration is present
• Provide outputs for Emergency Action
• Suitable for extreme environments (arctic, tropic, etc)
• Complement to Flame Detection equipment
• Cost-effective protection
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Gas Sensing TechnologiesGas Sensing Technologies
• Combustibles: Catalytic and Infrared• Hydrogen Sulfide: Electrochemical & MOS• Oxygen: Electrochemical• Carbon Monoxide: Electrochemical• Chlorine: Electrochemical• Sulfur Dioxide: Electrochemical• Nitrogen Dioxide: Electrochemical
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Key Gas Detection DefinitionsKey Gas Detection Definitions
• Measurement range: gas concentration at full scale sensor output
• Unit of Measurement: %LEL, PPM, or %volume• Flashpoint: minimum temperature of
liquid where enough vapor is given off to sustain a
fire or explosion• Vapor Density: tendency of a pure gas of vapor
to rise or sink in clean air• Calibration: process of matching transmitter
signal to sensor measurement
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Combustible Gases-Lower & Upper Limits*
Combustible Gases-Lower & Upper Limits*
Too lean for ignition Too rich for ignition
Clean Air
Combustibles
LowerExplosiveLimit (LEL)
UpperExplosiveLimit (UEL)
Typical CombustibleGas Detector Range
of Measurement
50%
*No differentiation between the terms “Explosive” and “Flammable” as applied to the lower and upper limits of flammability
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Flammable Limits & Volumetric Equivalents
Flammable Limits & Volumetric Equivalents
Gas Type L.E.L. U.E.L.Methane 5% 15%Propane 2.1% 9.5%Hydrogen 4.0% 75%Hexane 1.1% 7.5%
Methane Gas: 100% LEL= 5% by volume in air
75% = 3.75 “50% = 2.5% “
25% = 1.25% “ 10% = .50% “
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Catalytic Sensor OperationCatalytic Sensor Operation
• Contact-based• Destructive measurement• Traditional method• Only option for hydrogen gas
detection• Used w/ transmitter module
to provide 4-20 ma signal output
• Typical life of 3-6 years• 2 year mfgr warranty
DET-TRONICS
ALARM
INCREASE SETUP DECREASE
ACCEPT
CAL/RESET
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• Active Element - Hot wire resistor coated with catalytic material
• Reference Element - Same as active but without catalyst
• Catalytic Oxidation of flammable gas generates heat & changes resistance of Active Element
• Explosion is prevented by Flame Arrestor
• Constant U or I power source required
• Must calibrate on start-up & routinely for accuracy
Flammablegas
Catalytic Sensor OperationCatalytic Sensor Operation
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Catalytic Sensor OperationCatalytic Sensor Operation
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Catalytic Sensor LimitationsCatalytic Sensor Limitations
FAIL TO DANGER•Poisoning of Catalyst•Plugging of Flame Arrestor•Read Low @ High Gas Levels
•Frequent Calibration•Service Life - Variable (Months)
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Infrared Combustible DetectorsInfrared Combustible Detectors
• Principal of Operation• Gases are Transparent
to Visible Light BUT...• Gases are Strongly
Opaque to Infrared Light at Some Wavelengths
Methane
Propane
active absorption band
reference absorption band
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Point Infrared DetectorPoint Infrared Detector
• Principle of Operation
Microprocessorand
Electronics
GasConcentration
(LEL)
IR Source Perm eable GasCell
ReferenceSignal Detector
M easurem entSignal Detector
O ptica l B eam splitte r
O ptica l F ilte rs
IR T ransparent W indows
Flammablehydrocarbon
gas
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Fail-Safe OperationFail-Safe Operation
DirtyOpticsandCleanAir
A ctive R eference
IR S igna l Leve l
HydrocarbonGas
A ctive R eference
IR S igna l Leve l
CleanAir
A ctive R eference
IR S igna l Leve l
A ctive R eference
IR S igna l Leve l
FailedSensor
A ctive R eference
IR S igna l Leve l
FailedIR-Source
Failed M icroprocessor = 0 mA
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Open Path IR Gas DetectorOpen Path IR Gas Detector
• Instrumental Layout
IR Source
Microprocessorand
Electronics
GasConcentration
(LEL)
ReferenceSignal Detector
MeasurementSignal Detector
Optical Beamsplitter
Optical Filters
10 - 60 m
Flammablehydrocarbon
gas
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Drivers for Optical TechnologyDrivers for Optical Technology
• Safety Driven Design Requirements– FAIL SAFE
• No Unrevealed Sources of Failure– HIGH AVAILABILITY
• Long Service Life• Low and Easy Maintenance
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Electrochemical Toxic Sensor Theory
Electrochemical Toxic Sensor Theory
• Specific to toxic gas type (H2S, CO, SO2, NO2, CL2 )• Consists of sensing(working), reference, and counter electrodes• Vapor diffusion into capillary results in oxidation/ion reduction• The signal (current) from the working electrode is converted to a
voltage by operational amplifier
Capillary Entrance
O-ring
Membrane
Electrolytic fluid
Sensing Electrode
Reference Electrode
Counter Electrode
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Electrochemical Toxic Sensor Operation
Electrochemical Toxic Sensor Operation
• All provide 4-20 ma signal output
• 0-100 ppm range typical
• Carbon monoxide ranges: 0-100, 0-500, or 0-1000 ppm- must specify
• Nitrogen dioxide range: 0-20 ppm
• Hydrophobic filter provides humidity protection
• All designed to meet X-proof req. except CL2, SO2 sensors (I.S. barrier required)
• All designed for calibration by a transmitter, controller or external control system.
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Traditional Sensor CalibrationTraditional Sensor Calibration
• Matching transmitter output to raw sensor signal in clean air (zero) & mid-scale (span) conditions
• Manual or Automatic depending upon transmitter type
• Adjustments necessary if drifting has occurred• Requires compressed, accurate calibration gases• Always required @ start-up & after sensor
replacement• Every 30-90 days thereafter typical
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Gas Detector Calibration Requirements
Gas Detector Calibration Requirements
• Catalytic Sensors: Field Calibration required for accuracy• DEC IR Detectors: Routine calibration not required;
Factory calibrated for methane only• Electrochemical Sensors: Field Calibration required for accuracy• DEC Open Path IR System: Routine calibration not required;
Factory calibrated for methane only